This invention relates to sonic borehole logging tools and more particularly to a transducer assembly which directs acoustical energy at a preselected angle with respect to a borehole wall.
The following patents are believed to be relevant to the present invention: U.S. Pat. No. 3,974,476, issued to Cowles on Aug. 10, 1976; and U.S. Pat. No. 3,978,939, issued to Trouiller on Sept. 7, 1976.
The Trouiller patent provides a fairly complete statement of the prior art in the field of acoustic or velocity well logging. Such logging basically involves the injection of acoustic energy into a borehole wall at one point and the reception of part of that energy which has been transmitted back into the borehole at another point. The time lag between transmission and reception can be used to indicate acoustic energy velocity and other parameters. More accurate equipment has a single transmitting transducer and two receiving transducers with the time measurement being made between the reception at the two transducers which are spaced apart a known distance. The transducers which have typically been used are hollow cylindrical elements made of magnetostrictive or piezoelectric materials. Such elements are illustrated in FIG. 1 of the Trouiller patent. These elements transmit a majority of their acoustic energy straight out into the formation in a disc-shaped pattern perpendicular to the borehole axis. Such a pattern is illustrated in FIG. 4 of Trouiller. The energy which is thus transmitted straight out into the formation does not travel along the borehole wall where it can be received by receiving transducers. Thus, in the known devices only a small portion of the transmitted energy ever reaches the receiving transducer. As is known in the art and also illustrated by Trouiller in FIG. 5 of his patent, there is a critical angle of incidence at which acoustic energy impinging on the borehole wall will be refracted and travel through the borehole wall essentially parallel to the borehole. The ideal transducer would transmit 100% of its energy at or just below this critical angle so that essentially all of it would be traveling parallel to the borehole after incidence on the borehole wall. The Trouiller patent describes an improvement over prior art systems in which a solid cylindrical transducer element placed perpendicular to the borehole wall is used for both transmitting and receiving acoustical energy. As illustrated in FIG. 9 of Trouiller, such a cylindrical transducer is essentially omnidirectional, that is, transmits energy equally well in all directions perpendicular to its axis. Thus, Trouiller provides a transducer in which a significant amount of the transmitted energy does impinge upon the borehole wall at or near the critical angle. This is a significant improvement over the prior art cylindrical elements positioned parallel to the borehole wall in which only edge effect transmissions, which account for a very small part of the transmitted energy, actually impinge upon the borehole wall at the desired angle.
The Cowles patent illustrates a different type of borehole surveying tool. In this tool the energy is intentionally transmitted at right angles to the borehole wall, since reflection of energy from casing, borehole wall, fractures, etc., directly back into the same transducer is what is desired. The Cowles patent illustrates the use of an essentially flat disc-shaped transducing element and a flat reflecting element for directing energy into the borehole wall.
Thus, it is seen that in sonic logging tools of the type wherein acoustic energy is injected into a borehole wall to travel along the wall and is detected by one or more spaced receiving transducers, no transducers are available which direct substantially all of their acoustical energy at the borehole wall at or near the preferred critical angle.
Accordingly, an object of the present invention is to provide an acoustical energy transducer assembly for use in sonic logging in which substantially all of the transmitted acoustical energy is directed at a borehole wall at or near a critical angle.
Another object of the present invention is to provide an acoustic transducer assembly for borehole sonic logging which has essentially unidirectional response, that is, transmits or receives acoustical energy either up or down the borehole wall, but not both at the same time.